Superconductive magneto-resistive device for sensing an external magnetic field

ABSTRACT

A superconductive magneto-resistive device for use in a sensor system for sensing an external magnetic field which is formed so as to have a predetermined pattern for a current path through which a supplied current flows. The pattern includes portions formed close and parallel to each other so that magnetic fields induced by respective currents flowing through the portions can be cancelled with each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconductive magneto-resistivedevice for a magnetic sensor.

2. Description of the Prior Art

Conventionally, a magnetic sensor which utilizes the Hall effect onmagneto-resistive effect in a semiconductor or a magnetic sensor whichutilizes the magneto-resistive effect in a magnetic material is widelyused for sensing or measuring a magnetic field. The former sensor has asensitivity capable of sensing a magnetic field of about 10⁻² gaussesand the latter one has a sensitivity of about 10⁻³ gausses.

However, these conventional magnetic sensors have various disadvantagesas follows.

They have relatively large specific resistance R₀ even when no magneticfield is applied to them.

Each variation ratio of resistance to the magnetic field is representedby a parabolic curve having a small coefficient, as shown in FIG. 1qualitatively. Since a gain ΔR in the resistance is increasedproportional to the square of the magnetic flux density B of an appliedmagnetic field, the gain related to the application of a weak magneticfield of, for example, several tens of gausses is very small and,therefore, a ratio of the gain ΔR to the specific resistance R₀ (ΔR/R₀)is on the order of 1% at the most.

On the contrary, a magnetic sensor with use of the SQUID(Superconductive Quantum Interference Device) which utilizes theJosephson junction is known to have a very high sensitivity capable ofsensing a very weak magnetic field of about 10⁻¹⁰ gauss. Structures oftunnel junction, point contact and micro bridge have been known as theJosephson junction.

However, the magnetic sensor of this type has a quite delicate structurein manufacturing and requires a complicated operation to use it. Namely,it is not practical for general use although it has a very highsensitivity.

In a copending application (U.S. Ser. No. 226,067) which was filed inthe name of KATAOKA et al on Jul. 29, 1988 and will be assigned to SHARPKABUSHIKI KAISHA, a superconductive magneto-resistive device is proposedwhich is comprised of a superconductive material having grain boundariesacting as weak couplings and means for utilizing a change in theresistance of the material caused when a magnetic field is appliedthereto.

As shown schematically in FIG. 2, the superconductive material iscomprised of superconductive grains 1 and grain boundaries 2 bondingthem. These random grain boundaries 2 are considered or supposed to formvarious weak couplings 3 including tunnel junctions, point contactjunctions and micro bridge junctions, as shown by an equivalent networkcircuit of FIG. 3. In the superconductive phase thereof, individualCooper pairs can pass freely through weak couplings 3 (Josephsonjunction) and, therefore, the resistance becomes zero. When a magneticfield is applied to the superconductor, some of Josephson junction 3 arebroken thereby and, accordingly, the superconductor has an electricresistance. As a superconductor having grain boundaries, a Y-Ba-Cu-Oceramic superconductor can be used. The critical temperature thereof isabout 90 K.

FIG. 4 shows an example of the magnetic sensor system disclosed in theabove identified application.

In this system, an elongated rectangular device 4 of (1×7×0.7 mm³) whichis made of a Y-Ba-Cu-O ceramic superconductive material is prepared andis immersed in liquid nitrogen (77 K). A current is supplied by a powersource 9 through a pair of electrodes 5 and 6 formed on respective endsthereof and a voltage between two electrodes 7 and 8 is measured todetect a change in the resistance thereof when a magnetic field B isapplied thereto.

FIG. 5 shows the result obtained. As is apparent therefrom, theresistance of the device 4 changes according to the strength I of theapplied current and that of the applied magnetic field B. One of theadvantages of this system is that the specific resistance of the deviceis zero in the superconductive phase and another advantage is that thechange in the resistance of the device is very steep and, therefore, avery high sensitivity to the magnetic field is obtained.

However, in this system, there is a problem which is that the magneticsensor senses a magnetic field induced by the current flowing throughthe device because of the fine sensitivity thereof. In order to avoidthis problem, it is desirable to form the superconductive devicelinearly, as shown in FIG. 7. But, such a linear device induces amagnetic field proportional to the length thereof which causes an errorin the measurement of the strength of an external magnetic field to bemeasured.

SUMMARY OF THE INVENTION

One of the objects of the present invention is to provide asuperconductive magneto-resistive device for a magnetic sensor systemhaving a structure in which a magnetic field induced by a currentflowing through the device does not affect an external magnetic field tobe measured.

Another object of the present invention is to provide a magnetic sensorcapable of detecting a magnetic field having a non-dimensional ortwo-dimensional distribution.

In order to achieve these objects, according to the present invention,there is provided a superconductive magneto-resistive device for use ina sensor system for sensing an external magnetic field. A current issupplied to the device while keeping it at a temperature close to thecritical temperature of the superconductive material forming it. When anexternal magnetic field is applied thereto, the change in the resistancethereof caused by the applied magnetic field is detected in order tomeasure the applied magnetic field. The device is formed so as to have apredetermined pattern for a current path through which the suppliedcurrent flows; and that said pattern includes portions formed close andparallel to each other so that magnetic fields induced by respectivecurrents flowing through said portions can be cancel each other out.

According to another object of the present invention, there is provideda sensor system for sensing an external magnetic field comprising pluralsuperconductive magneto-resistive devices, supply means for supplying aconstant current to each of said devices, means for cooling said devicesat a temperature close to the critical temperature of a superconductivematerial forming each device, and detection means for detecting a changein the resistance of each device. The plural devices are arranged so asto form a predetermined pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIG. 1 is a graph showing the magneto-resistive property of aconventional magnetic sensor;

FIG. 2 is an enlarged schematic cross-sectional view of a ceramicsuperconductor for forming a superconductive magneto-resistive deviceaccording to the present invention;

FIG. 3 is an equivalent circuit for showing a network of weak couplingsformed in the ceramic superconductor shown in FIG. 2;

FIG. 4 is a perspective view showing a magnetic sensor disclosed in thecopending application of U.S. Ser. No. 226,067;

FIG. 5 is a graph showing the magneto-resistive property of the magneticsensor shown in FIG. 4;

FIG. 6 is a perspective view showing a magnetic sensor system accordingto the present invention;

FIG. 7 is an enlarged perspective view of the superconductivemagneto-resistive device used in the magnetic sensor system shown inFIG. 6;

FIG. 8 and FIG. 9 are plan views showing other examples of thesuperconductive magneto-resistive device, respectively;

FIG. 10 is a perspective view of a superconductive magneto-resistivedevice having a layered structure;

FIG. 11 is an enlarged cross-sectional view of another superconductivemagneto-resistive device according to the present invention;

FIG. 12 is a graph showing the magneto-resistive property of the deviceshown in FIG. 11;

FIG. 13(I), 13(II) and 13(III) are schematic plan views showing arraystructures of plural superconductive magneto-resistive devices fordetecting a pattern of an external magnetic field, respectively;

FIG. 14 is a schematic perspective view for showing a three-dimensionalarray of plural superconductive magneto-resistive devices for detectingthe strength and the direction of a magnetic field;

FIG. 15 is a side view of a magnetic sensor having a cooling meansaccording to the present invention;

FIG. 16 is a front view of a magnetic sensor suitable for detecting anorientation of an external magnetic field according to the presentinvention;

FIG. 17 is a front view of a superconductive magneto-resistive deviceused in the magnetic sensor shown in FIG. 16; and

FIGS. 18(a), 18(b) and 18(c) are explanative views for showing theaction of the magnetic sensor shown in FIG. 16.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 shows a magnetic sensor system according to the presentinvention.

The magnetic sensor system is comprised of a superconductivemagneto-resistive device 11 housed in a package 12 made of anon-magnetic material, a cooling apparatus 13 for cooling the device 11with use of high pressurized N₂ gas so as to keep it in thesuperconductive state, a circuit 14 for generating a constant current toapply it to the device 11, a detection circuit 15 for detecting avoltage generated in the device 11 and a microcomputer 16 forcontrolling the constant current generation circuit 14 and processingdata outputted from the voltage detection circuit 15. The processed datais displayed on a display 17.

When an external magnetic field B is applied to the device 11 in adirection indicated by an arrow B, the detection circuit 15 measures thevoltage generated in the device 11 and the measured voltage is processedby the microcomputer 16 in order to give the strength of the appliedmagnetic field.

FIG. 7 shows the superconductive magneto-resistive device 11.

The device 11 is comprised of a substrate 21 made of alumina (Al₂ O₃)and a superconductive magneto-resistive element 22 formed on thesubstrate 21. The element 22 is formed on the substrate 21 as a thinfilm of a Y-Ba-Cu-O ceramic superconductor with use of the spatteringmethod.

This element 22 has two linear portions 22a and 22b extending parallelto each other with a small distance which are connected by a connectionportion 22c at respective ends thereof. On respective free ends of twolinear portions 22a and 22b, electrodes 23a and 23b for supplying acurrent to the device 11 are formed by depositing Ti and lead lines 24aand 24b are connected to the electrodes 23a and 23b in order to applythe constant current from the constant current supplying circuit 14 tothe device 11. Further, a pair of electrodes 25a and 25b for measuring avoltage generated in the device 11 are formed on portions of the device11 near the current electrodes 23a and 23b. Two leads 26a and 26b fromthe voltage detection circuit 15 are connected to these electrodes 25aand 25b, respectively.

When a constant current I is applied to the current electrode 23a, itflows through the first linear portion 22a in a direction indicated byan arrows L and, then, returns through the second linear portion 22b tothe current electrode 23b, via the connecting portion 22c, in adirection indicated by an arrow M.

Since the distance between the first and second linear portions 22a and22b is very small, magnetic fields induced by the currents flowingoppositely through the first and second linear portions 22a and 22bcancel with each other. Therefore, the external magnetic field B to bedetected is not affected by these magnetic fields induced along thefirst and second linear portions 22a and 22b. Thus, the device 11 candetect the strength of the external magnetic field B exactly.

The superconductive element 22 is made with use of a superconductivematerial of a Y-Ba-Cu-O oxide having a critical temperature of 90 to 100K. This material is deposited on the substrate of Al₂ O₃ by spatteringin order to form a thin film of the thickness of about 10 μm. This filmis heated up to 900° C. in the air and, then, cooled gradually. Theobtained component thereof is Y₁ Ba₂ Cu₃ O_(7-x) (0<x<1). The film isprocessed by etching to form the element 22 on the substrate 21.

This film for the device can be made by various methods such as vacuumevaporation method, CVD method, spray method for spraying a solvent ofcomponents of the superconductive material and the like. A substratemade of silicon or Ba₂ TiO₄ is usable for the substrate of the device11.

The sensitivity of the superconductive magneto-resistive device isconsidered to be determined by a radius of grains included therein andthe state of grain boundaries.

The ceramic superconductive material can be also made by sintering asfollows.

Powders of Y₂ O₃, BaCO₃ and CuO are weighed at a predetermined ratio inorder to obtain a component of Y₁ Ba₃ Cu₃ O_(7-x) (0<x<1) After grindingand mixing these powders, samples formed with the mixture are calcinedat 900° C. for 5 hours in air. Then, the samples are crushed and groundinto powder comprised of micro particles having a diameter equal to orsmaller than 1 μm. Then, the powder is cold-pressed into samples.Finally, these samples are sintered at 1000° C. for 3 hours in air.

The sensitivity of the superconductive magneto-resistive device made bysintering as mentioned above is greatly dependent on the radius ofcrushed micro particles.

On the contrary to the sintering method, the diameter of grains formingthe superconductive film made by the deposition method is substantiallydetermined by the temperature of the substrate upon depositing the filmthereon.

In the preferred embodiment, the ceramic superconductive film is formedby spattering the material on the substrate while keeping it at atemperature of 300° to 400° C. The deposited film to sintered at 950° C.in air and, thereafter, cooled gradually.

The pattern of the device can be formed by irradiating a laser beam, anelectron beam or an ion beam onto portions of the film except for thepattern in order to change those into a normal conductive state.

FIG. 8 and FIG. 9 show desirable patterns for the device.

The pattern shown in FIG. 8 has successive four basic patterns shown inFIG. 7 which are formed parallel. This pattern has a length of currentpath of four times of that of the basic pattern. Accordingly, an outputvoltage of four times can be obtained in this pattern with the samecurrent. The number of basic patterns can be changed arbitrarily.

The pattern shown in FIG. 9 is formed to have five parallelized portionsconnected one after another. This pattern has a current path of aboutfive times of that of the basic pattern.

The pattern having a structure for cancelling magnetic fields induced byrespective linear portions can be realized not only by a plane patternbut also by a stacked or layered structure.

FIG. 10 shows an example of such a stacked structure.

In this example, the device 31 is comprised of first and second elements32 and 33 and an insulation film 34 inserted inbetween them.

Each of the first and second elements 32 and 33 has a linear pattern ofa superconductive magneto-resistive material formed on each ofsubstrates 35 and 36, as indicated by a dotted line in FIG. 10. Thepatterns of the first and second elements 32 and 33 are formed identicalwith each other.

Each one end of these patterns of the first and second elements 32 and33 are electrically connected with each other by a through hole 37formed on the insulation film 34.

On the other ends of these patterns, electrodes 38 and 39 are,respectively, formed for supplying a constant current from the constantcurrent circuit 14. The electrode 38 is drawn out through a through hole40 to the upper surface of the first substrate 35.

In this structure, the direction of the current flowing through thedevice is reversed between the pattern of the first element 32 and thatof the second element 33 and, therefore, respective magnetic fieldsinduced along the current paths of the first and second elements 32 and33 are perfectly cancelled with each other.

FIG. 11 shows another example of the device 50 having a layeredstructure.

In this structure, six layers from 51 to 56 of a superconductivemagneto-resistive material are deposited one by one and, betweenadjacent layers, an insulation layer 57 is formed so as to insulate themexcept for one end portion of them. The insulation layer 57 forinsulating the upper pair of the adjacent layers is formed so as toextend in a direction opposite to that of the lower pair of the adjacentlayers and, thus, a folded current path is formed in the device 50. Onthe lower-most and upper-most layers 51 and 56, a pair of electrodes 58aand 58b for supplying a constant current I from the current source 14and a pair of electrodes 59a and 59b for measuring a voltage generatedin the device 50 by the detection circuit such as a potentiometer 15 areformed, respectively.

Since the current is reversed in the direction thereof between theadjacent layers, magnetic fields induced by respective currents flowingthrough the adjacent layers are perfectly cancelled with each other.

This structure is extremely advantageous in that the output voltage orthe resistance to be measured is independent from the strength of thecurrent to be applied to the device because no internal magnetic fieldis generated in the device 50 and, accordingly, the resistance of thedevice is determined only by the external magnetic field appliedthereto.

FIG. 12 is a graph showing the result obtained by measurement with useof the device 50 having the structure shown in FIG. 11. Themagneto-resistive characteristic obtained when the current of 0.1 mA issupplied substantially coincides with that obtained when the current of0.01 mA is supplied.

FIGS. 13(I) and 13(II) show one dimensional magnetic array sensor andtwo dimensional magnetic array sensor, respectively.

In the one dimensional magnetic array sensor, a plurality ofsuperconductive magneto-resistive devices from 61-l to 61-n areconnected parallel to each other between lines 62 and 63 connected to apower source.

To each of the devices, a resistance 64 is connected serially and eachoutput terminal 65 is drawn out from a portion between each device andeach resistance.

When a magnetic field having one dimensional pattern is applied to thesensor, the pattern is detected based upon data outputted fromindividual devices 61-l to 61-n of the sensor. The device having such apattern as shown in either of FIGS. 7 to 11 is desirably used for eachdevice of the sensor. However, the superconductive magneto-resistivedevice as shown in FIG. 4 can be used for the device of the sensor.

In the two dimensional magnetic array sensor shown in FIG. 13(II), aplurality of superconductive magneto-resistive devices are arranged in amatrix form.

In this case, a two dimensional magnetic pattern can be detected basedon data outputted from individual devices.

FIG. 13(III) shows another example of the two dimensional magneticsensor.

In this magnetic sensor, plural column line devices 71-l to 71-n areformed on a substrate (not shown) at a predetermined pitch and pluralrow lines devices 81-l to 81-n are formed at a predetermined pitch so asto form a lattice together with the column line devices 71-l to 71-n.Each of the row line devices and each of the column line devices areinsulated with each other at the crossing portion between them.

Individual one ends of the column line devices are connected to a powersource line 75 and individual one ends of the row line devices areconnected to another power source line 85. The other end of each of thecolumn line devices is connected to each of other source terminals 76-lto 76-m via a resistance 77. Also, the other end of each of the row linedevices is connected to each of other power source terminals 86-l to86-n via a resistance 87. Each of output terminals 78-l to 78-m is drawnout from a portion between each of the column line devices and theresistance 77. Also, each of output terminals 88-l to 88-n is drawn outfrom a portion between each of the row line devices and the resistance87.

When a magnetic field as indicated by a dotted circle H is applied tothe sensor, only two output terminals of the second column lineterminals 78-2 and the second row line terminal 88-2 output datacorresponding thereto. Accordingly, a two dimensional magnetic field canbe detected by scanning the column and row line terminals sequentially.

It is also possible to apply the power to an arbitrary pair of thecolumn line device and the row line device by providing respectiveswitching means for selectively switching on either of the column linedevices and for selectively switching either one of the row linedevices. If a pair of i-th column line device 71-i and j-th row linedevice 81-j are switched on, the magnetic field induced by a currentflowing through either one of them is applied to the other device as abias magnetic filed reciprocally. Due to this, it becomes possible todetect an external magnetic field applied to one of crossing pointsselectively by the application of the internal bias magnetic fieldthereto.

In FIG. 14, a three dimensional magnetic sensor is disclosed. In thissensor, three superconductive magneto-resistive device 91, 92 and 93 arearranged along three orthogonal coordinate axes X, Y and Z,respectively.

When an external magnetic field H_(M) is applied to the magnetic sensor,the direction and the strength thereof can be calculated based onrespective output data from individual devices 91, 92 and 93.

FIG. 15 shows a cooling apparatus for the device 11 utilizing Peltiereffect which is formed as a cascade structure of two stages with use ofPeltier effect devices.

In FIG. 15, reference numeral 41 denotes a heat radiation metal plate,reference numeral 42 denotes a cooling metal plate, reference numeral 43denotes an insulator, reference numerals 44a and 44b denote p-type andn-type semiconductor devices, respectively, and reference numeral 45denotes a heat radiation substrate.

FIG. 16 shows another example of a magnetic sensor 101.

This sensor 101 is comprised of two rod-like elements 102 and 102' madeof a material having a high permeability and a superconductivemagneto-resistive device 103 inserted between two elements 102 nd 102'.As shown in FIG. 17, the device 103 is comprised of a substrate 105 anda folded linear pattern element 106 of a superconductivemagneto-resistive material deposited thereon. A constant current isapplied from a pair of electrodes 107a and 107b formed on respective endportions of the pattern element 106.

When the magnetic sensor 101 is directed parallel to the magnetic fluxof a magnetic field as shown in FIG. 18(a), the magnetic flux isconverged into rod-like element 102 or 102' and, therefore, a strongmagnetic field is applied to the device 103 to generate a highresistance in the device.

On the contrary, if the magnetic sensor 101 is inclined to the magneticflux of a magnetic field by an angle α as shown in FIG. 18(b), themagnetic flux is not converged so much into the rod-like element 102 and102'. Therefore, a magnetic field applied to the device 103 becomesconsiderably weak. And, if the magnetic sensor 101 is directedperpendicularly to the magnetic flux of a magnetic field, as shown inFIG. 18(c), all of the magnetic flux pass freely through each of therod-like elements 102 and 102' and, therefore, none of the magneticfield is applied to the device 103. Thus the direction of a magneticfield relative to the magnetic sensor can be detected based on dataoutputted from the device 103.

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of the present invention. Accordingly, it isnot intended that the scope of the claims appended hereto be limited tothe description as set forth herein, but rather that the claims beconstrued as encompassing all of the features of patentable novelty thatreside in the present invention, including all features that would betreated as equivalents thereof by those skilled in the art to which thepresent invention pertains.

What is claimed is:
 1. A superconductive magneto-resistive device forsensing the direction of a magnetic field applied thereto comprising:asuperconductive magneto-resistive element being formed of a materialincluding grain boundaries having weak couplings and being configured toform a predetermined continuous current pattern; two rod-like elementsmade of a material having a high permeability; and said superconductiveelement being located between said two rod-like elements.
 2. The methodof measuring the direction of magnetic field comprising the followingsteps:(a) taking a device formed of a superconductive magneto-resistiveelement which has been placed between two rod-like elements made ofmaterial having a high permeability; (b) placing said device in themagnetic flux of said magnetic field; and (c) measuring any change inresistance in said device.
 3. The method of claim 2 wherein said deviceis placed parallel to the direction of the magnetic flux of saidmagnetic field.
 4. The method of claim 2 wherein the device is placed atan angle to said magnetic flux of the magnetic field.
 5. The method ofclaim 2 wherein the device is placed perpendicular to the magnetic flux.6. The method of claim 2 wherein said superconductive magneto-resistiveelement contains grain boundaries.
 7. The method of claim 6 wherein saiddevice is placed substantially perpendicular to the magnetic flux. 8.The method of claim 6 wherein said device is placed substantiallyparallel to the direction of the magnetic flux of said magnetic field.9. The method of claim 6 wherein said device is palced at an angle tosaid magnetic flux of the magnetic field.